Flammability of some ornamental species in
wildland–urban interfaces in southeastern France:
laboratory assessment at particle level
A. Ganteaume, M. Jappiot, C. Lampin, M. Guijarro, C. Hernando
To cite this version:
A. Ganteaume, M. Jappiot, C. Lampin, M. Guijarro, C. Hernando. Flammability of some
ornamental species in wildland–urban interfaces in southeastern France: laboratory assessment
at particle level. Environmental Management, Springer Verlag (Germany), 2013, 52, p. 467 p. 480. .
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Author-produced version of the article published in Environmental Management, 2013, vol. 52, pp 467-480
Original publication available at http://www.springer.com
doi : 10.1007/s00267-013-0067-z11
Flammability of some ornamental species in wildland-urban interfaces in Southeastern
France: laboratory assessment at particle level.
Anne GanteaumeA,C, Marielle JappiotA, Corinne LampinA, Mercedes GuijarroB, Carmen HernandoB
A
Irstea. UR EMAX, 3275 route de Cézanne, CS 40061, 13182 Aix-en-Provence, France
B
INIA. Forest Research Centre. Dpt. of Silviculture and Forest Management. Ctra. A Coruña km 7.5. 28040
Madrid, Spain
C
Corresponding author: [email protected]
Tel.: +33442666979
Fax: +33442669923
1
Author-produced version of the article published in Environmental Management, 2013, vol. 52, pp 467-480
Original publication available at http://www.springer.com
doi : 10.1007/s00267-013-0067-z22
Abstract
Assessment of the flammability of ornamental vegetation (particularly hedges) planted around houses is
necessary in light of the increasing urbanization of the wildland-urban interfaces (WUIs) and the high fire
occurrence in such areas. The structure and flammability of seven of the species most frequently planted as
hedges in Provence (Southeastern France) were studied at particle level. Spatial repartition of the different types
of fuel particles within plants was assessed by means of the Cube Method. The leaf flammability was assessed
using an epiradiator as a burning device and measurements of foliar physical characteristics and gross heat of
combustion (GHC) helped to explain the results of burning experiments. Co-inertia analysis revealed that species
with thin leaves were quick to ignite (Pyracantha coccinea, Phyllostachys sp.) and species with high leaf GHC
burned the longest (Pittosporum tobira, Nerium oleander). Species presenting high ignitability (Photinia
fraseri, Phyllostachys sp. and Pyracantha coccinea) were characterised by high foliar surface area-to-volume
ratio, and species presenting lower ignitabilitywere characterised by high GHC (Pittosporum tobira, Nerium
oleander, Cupressus sempervirens). Hierarchical cluster analysis of the flammability variables (ignition
frequency, time-to-ignition and flaming duration) categorized the relative flammability of the seven species
(including dead Cupressus sempervirens) in five clusters of species from poorly flammable (Pittosporum tobira)
to extremely flammable (dead Cupressus sempervirens).This study provides useful information for reducing fire
risk in WUIs in the study area.
Additional Key-words: ORNAMENTAL VEGETATION, FLAMMABILITY PARAMETERS, PLANT
PARTICLES, FIRE HAZARD, WILDLAND-URBAN INTERFACE
1.
Introduction
Around the world, concerns about the impact of wildland fires are increasingly focusing on the wildland-urban
interface (WUI), where their occurrence is high and where they can affect people and structures (Etlinger and
Beall, 2004). Fires in wildland-urban interfaces regularly destroy dwellings when fuel and weather are conducive
to fire (Covington 2000) and human-caused fire ignitions are most common in these areas (Cardille et al. 2001;
Jappiot et al. 2001; Lampin et al. 2006a, 2006b). As a result, WUIs are now considered as priority areas for
controlling wildfire (Stephens 2005). In the Mediterranean region, the incidence of fire is often higher in
wildland-urban interfaces and in wildland/network interfaces because of high urban pressure and accumulation
of wildland biomass (Davis 1990; Vélez 1997; Cohen 2000; Leone et al. 2003; de la Riva and Pérez-Cabello
2005; de la Riva et al. 2006; Jappiot et al. 2007; Lampin-Maillet 2009). Furthermore, the higher incidence of
extreme climate events (very high summer temperatures, strong winds and drought periods) expected under
climate change, together with the high flammability of Mediterranean fuels, implies higher probability of
ignition (Valette 1990; Dimitrakopoulos and Papaioannou 2001).
The presence of ornamental vegetation around houses is one of the major concerns regarding the
vulnerability of structures as this vegetation can act as “ladder fuel” and ignite houses and other structures
(Etlinger and Beall 2004). Therefore, less flammable species are recommended as ornamental plants (Monroe et
al. 2003). According to Dimitrakopoulos (2001) and Dimitrakopoulos and Papaiannou (2001), classification of
fuel in relation to its expected flammability is an essential component of fire hazard assessment. Homeowners in
WUIs are often advised to minimize or eliminate the use of highly flammable vegetation when landscaping their
homes, and lists containing species that are appropriate for use in fire-wise landscaping are often requested.
Reducing the fire hazard associated with vegetation can be accomplished by appropriate arrangement,
maintenance and selection of species. However, Lubin and Shelly (1997) highlighted many discrepancies in the
recommendations for plant selection by species, which often relied on anecdotal information with little scientific
basis, largely because of the scarcity of scientific data on the flammability of ornamental vegetation.
Nonetheless, there is an increasing demand from forest managers, fire managers and fire-fighters for
scientifically based information about ornamental species for use in WUIs.
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Author-produced version of the article published in Environmental Management, 2013, vol. 52, pp 467-480
Original publication available at http://www.springer.com
doi : 10.1007/s00267-013-0067-z33
Although losses of WUI vegetation and structures are substantial and are increasingly common, studies
on fire performance of ornamental plants, performed mainly in USA (White et al. 1996; Irby et al. 2000; Beall
2001; White et al. 2002; Monroe et al. 2003; Etlinger and Beall 2004; Weise et al. 2005), have not been carried
out in Europe. Some of the early studies sought to identify slow-burning plants that could survive in southern
California, e.g. plants with high mineral content that could act as fire-retardant plants (Ching and Stewart 1962)
and plant species with low fuel volume (Nord and Green 1977). More recent studies (Etlinger 2000; Etlinger and
Beall 2004) have focused on the relationship between plant characteristics and flammability components as
defined by Anderson (1970) and Martin et al. (1994).
Plant flammability has been widely studied and experimentally assessed in the laboratory for various
purposes, following several methods which took into account many different definitions and flammability
parameters. Moreover, methods which allowed the assessment of vegetation flammability and the results
obtained differed depending on the scale considered (Etlinger and Beall, 2004; Madrigal et al., 2012). However,
the combination of different methods for assessing the flammability parameters of ornamental vegetation as well
as the description of its structure, have not been studied yet.
In WUIs, two types of fire ignition can occur regarding ornamental vegetation: (i) ignition of the
vegetation directly subjected to the intensity of the flame front, as assessed in the present study (flammability at
particle level), and (ii) in case of spot-fire, ignition of the litter (ignition of dead surface fuel) by a glowing
firebrand that lands at the foot of the plant. The flammability of dead surface fuels, i.e. litter, that could be
ignited during a spot fire, has been assessed in a previous study (Ganteaume et al. 2012).
The objective of this work was to compare the potential flammability of some ornamental plants that
could be used as hedges for instance in Provence (SE France), taking into account the plant structure, some
variables of flammability and the gross heat of combustion of fine particles. According to the classical definition
(Anderson, 1970), flammability was considered as the result of ignitability (time until ignition), combustibility
(rapidity of the combustion after ignition) and sustainability (ability to sustain combustion once ignited).
Therefore, live leaf flammability was assessed by determining time-to-ignition, flaming duration and ignition
frequency.
2.
Material and methods
2.1. Study area and selection of species
Bouches du Rhône is one of the French “départements” which is located in eastern Provence (Fig. 1) and is one
of the areas most seriously affected by wildfires in SE France (forest fire database Prométhée,
www.promethee.com). The climatic conditions differ in the coastal fringe and in the inland part of the
département (Météo France database, www.meteofrance.com); thus, the species planted as hedges will also
differ depending on the location. Therefore, the study area was divided into two sub-areas (coastal and inland)
for the purposes of sampling. To identify the main species used as ornamental vegetation, a survey of hedges
was made in nine locations in both areas, taking into account a total of 117 hedges in four coastal locations and
110
hedges
in
five
inland
locations
(Fig.
1).
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doi : 10.1007/s00267-013-0067-z44
1
FRANCE
Fig. 1 Map of the study area (Département des Bouches du Rhône) in Southeastern France showing where the hedges were surveyed (BD Carto). White spots: coastal area, black
spots: inland area
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doi : 10.1007/s00267-013-0067-z55
A total of 20 species were recorded during the survey and were considered representative of the species
planted in the whole study area. Seven of these species were chosen for the present study, either because they
were the most frequent species planted as hedges (Prunus laurocerasus, Pyracantha coccinea, Cupressus
sempervirens, Nerium oleander, Photinia fraseri and Pittosporum tobira) or because of their singularity, like
Phyllostachys sp., a type of bamboo, which may have particular flammability characteristics.
2.2. Field samplings
The species were sampled in summer (between July and August 2011) during the fire season, when the fuel
moisture content is lowest and the fire risk is highest.
Fuel sampling at particle level
The sampling of particles (mainly leaves and twigs) was performed according to the “cube method” (Cohen et
al. 2002). This method allowed us to determine the structure of the plant (proportion and spatial distribution of
live and dead fuel particles of different sizes within the plant canopy) by use of a 0.008 m3 cube (20 x 20 x 20
cm). The cube was placed on the plant, at breast height, at the base, the centre and the top of the plant and the
biomass filling the cube was sampled (Fig. 2). For each species, the sampling was replicated in three different
plants in the hedge.
Photo : Cemagref
Fig. 2 Shape of the plant designed by three locations of cube (top, centre and base) and biomass sampling in a
Cypress “top” cube
Sampling of leaves for flammability experiments
To characterise the flammability of the live fuel according to the protocol proposed by Valette (1990), the leaves
of each species were sampled homogeneously, and only leaves of the same age and size were collected. Because
of the high proportion of dead fuel sampled within the canopy of Cupressus sempervirens, dead leaves were also
sampled for this species. To ensure that the fuel moisture content (FMC) was minimal, in order to be in the worst
case scenario in term of fire risk, each species was sampled in summer at the hottest time of the day (between 12
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Author-produced version of the article published in Environmental Management, 2013, vol. 52, pp 467-480
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am and 2 pm) and the sampling was not carried out on days following rainfall events (Valette 1990). The leaves
sampled were placed in plastic bags and stored in a cool box for transportation to the laboratory, minimizing
changes in water content.
Before burning, the physical characteristics of the live leaves of the different species (mass, surface,
volume and surface-to-volume ratio) were measured because the importance of particle geometry in determining
the combustion of fuel particles. To characterize the leaf surface exposed to hot gases and fire during
combustion, leaf thickness was measured avoiding thick veins. Leaf surface area was measured using a 2400 dpi
scanner and image analysis software. Particle thickness was measured using a 10-4m accuracy micrometer.
2.3. Description of laboratory protocols
Sorting of particles sampled by the “cube method”
In the laboratory, the samples were oven-dried (48h at 60°C) until the dry weight did not change (FMC <5%),
then the particles were sorted into four classes and weighed: (i) leaves, (ii) particles < 2 mm in diameter, (iii)
particles 2-6 mm, and (iv) particles > 6 mm. Particles were mainly composed of twigs and fractions of fruits.
Very fine fuel was composed of leaves and particles < 2 mm and fine fuel was composed of leaves and particles
up to 6 mm in diameter. Dead particles, that may affect the flammability, were separated from live particles in
order to have available both proportions within a species. Bulk density was calculated as the dry weight of
particles divided by the volume of the cube and expressed in kg m-3. Bulk density was used to refer to the
amount of fuel in a volume of airspace, for each class of particle, each cube location and each species.
Flammability experiments with the epiradiator
The epiradiator device consisted of a 500 W electric radiator with a 10 cm diameter radiant disk. The surface
temperature achieved with the device at steady-state regime was 420 °C (Fig. 3). Fifty samples (each weighing
1g) of live leaves of each species studied (and dead leaves of Cupressus sempervirens) were exposed to the heat
source (Valette 1990). Use of larger fuel masses may increase the possibility that other fuel properties, such as
fuel height on the epiradiator, would cause differences in flammability (Hernando 2000; Petriccione et al. 2006;
Pellizzaro et al. 2007). The samples were in direct contact with the electric radiator, and the surface area of
contact depended on the species. The surface in contact with the epiradiator was assumed to be close enough to
the heat source to undergo homogeneous heat transfer effects. A pilot flame, which did not take part in decay of
the sample, was located 4 cm above the centre of the disk and allowed more regular ignition of the gases emitted
during combustion of the leaf. The device was placed under a hood to prevent air currents perturbing the
convection column and gas plumes. Once the leaf samples were placed on the electric radiator, time to ignition
(TTI) and flame extinction were recorded to enable calculation of the flaming duration (FD) of the sample. The
ignition frequency (IF) was calculated as the percentage of tests in which the samples ignited. Each species was
classified according to its ignition frequency and its mean time to ignition following the classification outlined
by Valette (1990) which combined ignition frequency ranging from <50% to >95% and time-to-ignition ranging
from <12.5s to >32.5s and then rated the species from 0 to 5 ( 0 = slightly flammable; 1 = weakly flammable; 2
= moderately flammable; 3 = flammable; 4 = highly flammable; 5 = extremely flammable).
Before the experiments, a 5 g sample (fresh weight) of each species was oven-dried for 24h at 60°C to
enable calculation of the FMC according to the following equation:
FMC (%) = (Fresh weight (g) – Dry weight (g) )/ Dry weight (g) * 100
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Fig. 3 Epiradiator used in the flammability experiments
Gross heat of combustion of very fine fuel
Gross heat of combustion (GHC) of the very fine live fuel 1 (composite samples of leaves and particles <2 mm in
diameter formed from all cubes collected within a species) was determined following Spanish Standard UNE
164001 EX (Asociación Española de Normalización y Certificación 2005). All fuel samples were ground
individually to 5.10−4 m in a mill. A hand press was used to make the ground material into 1 g-pellets, which
were then oven-dried at 100±5°C for 24 h and weighed. Measurements were made with an IKA®C5000
adiabatic bomb calorimeter with a platinum resistance sensor (PT-100). Both mill and hand press were also
manufactured by IKA®. For each type of particle, the same measurements were made on two samples. A third
sample was included whenever the difference between the first two values was more than 2% of the mean value.
2.4. Data analyses
The effects of the species and of the cube location (sampled at the base, centre or top of the plant) on the
proportions of the different classes of particle and on the flammability variables (TTI, FD, IF) were analyzed by
two-way and one-way (when non-parametric tests were used) analyses of variance (ANOVA). In addition to the
tests of overall significance with ANOVA, the LSD test was used to check for significant differences between
the different means. A significant difference between the variables was assumed when the p-value was ≤ 0.05.
Some of the variables were previously log-transformed to meet ANOVA assumptions of normality and
homoscedasticity. In all cases, when the data distribution did not follow the expected parametric pattern or when
a test condition provided insufficient data, a non-parametric Kruskal-Wallis test was used instead of a parametric
Fisher test. All species and cube locations were tested at random as well as their interaction. During the
experiments, the mean relative temperature was 23.6°C and mean relative humidity was 52% but these variables
did not affect flammability (Fisher’s LSD test, p>0.05). All analyses were performed using Statgraphics
Centurion XV.
Co-inertia analysis (Dolédec and Chessel 1994), suited to large number of variables compared with
small number of samples, was performed on the dependent variables (flammability variables) and on the
explanatory variables (leaf traits and leaf GHC) to examine associations between leaf characteristics and leaf
flammability. The complete matrix of data was transferred to the statistical package under R 2.5.1 (R
Development Core Team, 2005) then analyzed using the ADE-4 package (Thioulouse et al. 1997). Co-inertia is a
statistical method commonly used to analyze the relationship between species and environmental variables (e.g.
Moretti and Legg 2009). The first step of the co-inertia analysis (Ter Braak and Schaffers 2004) was to conduct a
correspondence analysis (CA) on the leaf characteristics, then a principal component analysis (PCA) on the
flammability variables. A factorial plane was thus created and enabled a new ordination of each data set. The
1
The GHC of the very fine dead fuel was also measured for Cupressus sempervirens because of the high fraction
of dead fuel recorded in this species.
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statistical significance of each effect or combination of effects has been tested using a Monte-Carlo permutation
test with 1000 permutations using the ‘coin’ package on R. High sums of eigen values on the main axes indicate
high correlation among datasets. Hierarchical cluster analysis of the flammability variables was performed to
rank the seven species (including dead Cupressus sempervirens) according to their flammability. These
multivariate analyses were performed with R software (R 2.11-1, ADE-4 1.5-1 package).
3.
Results
3.1. Cube Method
The bulk densities of the most flammable classes of particles (leaves and particles < 2 mm) sampled in each cube
location for the seven species studied are shown in Table 1. Regarding live particles, the highest bulk densities
were located in the top and centre cubes. Cupressus sempervirens and Prunus laurocerasus presented the highest
values (respectively 7.87 and 6.31 kgm-3 in total) contrary to Pyracantha coccinea and Phyllostachys sp.
(respectively 0.96 and 0.93 kgm-3 in total). Regarding dead particles, Cupressus sempervirens presented huge
amount of dead leaves, especially located in base cubes (10.36 kgm-3 for a total of 11.12 kgm-3). There was no
dead fuel in the cubes sampled in Nerium oleander, Photinia fraseri and Phyllostachys sp. ; bulk densities of
dead particles were very low in the other species (< 0.15 kgm-3 in total) .
The species and the cube location significantly affected the proportions of live leaves, particles <2mm
and >6mm; the proportion of 2-6 mm particles was only affected by the species. When it was significant, the
cube location was the most important factor (Two-way ANOVA, Tab. 2). Regarding live fuel, the proportion of
leaves in Pyracantha coccinea was significantly lower than that in the other species but the proportion of 2-6
mm particles was the highest in this species. The proportion of particles < 2 mm in Phyllostachys sp. was
significantly higher than that in the other species. The proportion of particles >6mm was significantly higher in
Prunus laurocerasus and Nerium oleander than in Phyllostachys sp., Cupressus sempervirens and Pyracantha
coccinea (Fig. 4). The proportions of leaves, particles <2mm and >6mm significantly differed between the cube
locations (Fig. 5); top cubes presenting the highest proportions of leaves and particles <2mm and base cubes the
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Fig. 4 Proportions of the different classes of particle in the seven species studied (Phyllos.: Phyllostachys sp.). Different letters on the same bar indicate significant differences
according to two-way ANOVA Fisher test for live particles and one-way ANOVA Kruskal-Wallis test for dead particles
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Fig. 5 Proportions of the different classes of particles in the three cube locations. Different letters on the same bar indicate significant difference according to two-way
ANOVA Fisher test for live particles and one-way ANOVA Kruskal-Wallis test for dead particles
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other highest proportion of particles >6mm. Centre cubes presented higher proportion of 2-6mm particles than
the location (Fig.5). The proportions of live and dead particles that differed significantly between species are
presented in Figure 4 and the proportions of live and dead particles that differed significantly between cube
locations are shown in Figure 5. Regarding live fuel, the proportion of leaves was significantly higher in the top
cubes and regarding dead fuel, the proportion of very fine particles was significantly higher in the base cubes.
The interaction between the species and the cube location was significant in all the classes of particles
except in the >6mm class (Two-way ANOVA, Tab. 2). Regarding live leaves, the effect of the cube location was
not significant for Pyracantha coccinea and Cupressus sempervirens and it was significant only for Photinia
fraseri regarding the particles<2mm. Regarding the 2-6mm particles, the species significantly interacted with the
top cubes only (Fig. 6).
The species had always a significant effect on the proportions of the different classes of dead particles
while the cube location significantly affected the proportion of 2-6 mm particles (One-way ANOVA, Tab. 2).
Regarding dead fuel, cubes sampled in Cupressus sempervirens presented significantly higher proportions of
very fine fuel (leaves and particles < 2 mm) than the other species.
3.2. Flammability experiments with the epiradiator
The flammability experiments performed on the live leaves of the different species showed that Phyllostachys
sp. and Photinia fraseri were the most flammable species (short time-to-ignition, high ignition frequency and
flaming duration), while Pittosporum tobira was the least flammable species (long time-to-ignition and low
ignition frequency) according to Valette’s classification (1990). Results also showed that, regarding live leaves,
Cupressus sempervirens was among the least flammable species despite its high ignition frequency (Table 3) but
the dead leaves of this species were the most flammable particles (IF = 100%, TTI < 3s and FD > 15s),
regardless of the flammability variables and of the species. Because of this, data on dead leaves have not been
taken into account in the ANOVAs.
ANOVAs performed on the data recorded during these experiments showed that the species had a
highly significant effect on flammability variables obtained with this experimental device, especially on time-toignition (Table 3). The ignition frequency of Pittosporum tobira leaves was significantly lower than that of
leaves of the other species, while the ignition frequency of Photinia fraseri leaves was significantly higher than
that of Nerium oleander and Pyracantha coccinea leaves. Furthermore, Prunus laurocerasus leaves ignited more
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Fig. 6 Interactions (and 95% Fisher LSD interval) between cube location and species for the different classes of live particles (Phy: Phyllostachys sp., Cu: Cupressus sempervirens, Ne:
Nerium oleander, Pho: Photinia fraseri, Pi: Pittosporum tobira, Pr: Prunus laurocerasus and Py: Pyracantha coccinea)
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frequently than those of Nerium oleander. The time-to-ignition was shorter for Phyllostachys sp. leaves than for
the the others, while Cupressus sempervirens leaves took the longest time to ignite. The time-to-ignition did not
vary significantly between Photinia fraseri and Pyracantha coccinea, or between Prunus laurocerasus and
Pyracantha coccinea. Prunus laurocerasus and Cupressus sempervirens leaves burned significantly faster than
that of the other species, but the flaming duration of Cupressus sempervirens leaves was not significantly lower
than that of Pyracantha coccinea leaves.
3.3. Gross heat of combustion
The gross heat of combustion (GHC) of very fine fuels (measured by calorimetry) was the highest in Nerium
oleander and Photinia fraseri leaves and the lowest in Phyllostachys sp. leaves (Table 5). The GHC of twigs < 2
mm in diameter in this latter species was the highest and it was the lowest in Prunus laurocerasus. The species
had a significant effect on GHC of twigs <2 mm in diameter, but not on GHC of leaves (Table 4). GHC of twigs
<2 mm in diameter did not differ between Nerium oleander and both Pittosporum tobira and Prunus
laurocerasus, nor between Pittosporum tobira and Prunus laurocerasus as well as between Photinia fraseri and
Pyracantha coccinea.
3.4. Classification of species
The cloud plot extracted from the co-inertia analysis (Fig. 7) showed the position of the dependent variables
(flammability variables) and of the explanatory variables (leaf traits and GHC). Axis 1 explained 58% of
variance and differentiated species presenting high ignitability (Photinia fraseri, Phyllostachys sp. and
Pyracantha coccinea), characterised by high foliar surface area-to-volume ratio, from species presenting lower
ignitability characterised by high GHC (Pittosporum tobira, Nerium oleander, Cupressus sempervirens). Axis 2
explained 42% of variance and opposed species with short flaming duration (Cupressus sempervirens and
Prunus laurocerasus), characterised by thick leaves and low foliar surface area-to-volume ratio with species
presenting higher sustainability and ignitability (Photinia fraseri, Phyllostachys sp.).
Hierarchical cluster analysis of the flammability variables (ignition frequency, time-to-ignition and
flaming duration) categorized the relative flammability of the seven species (including dead Cupressus
sempervirens) in five clusters of species (Fig. 8): (i) poorly flammable (Pittosporum tobira), (ii) not very
flammable (live Cupressus sempervirens), (iii) moderately flammable (Pyracantha coccinea, Nerium oleander
and Prunus laurocerasus), (iv) highly flammable (Photinia fraseri and Phyllostachys sp.) and (v) extremely
flammable (dead Cupressus sempervirens).
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Fig. 7 Results of the co-inertia analysis performed on the leaf traits (M: mass, S: surface area, V: volume, S/V:
surface area to volume ratio, T: thickness) and Gross Heat Content (GHC) values as explanatory variables and
on the flammability variables (TTI: time-to-ignition, FD: flaming duration, IF: ignition frequency) as dependent
variables giving the position of species (Phy: Phyllostachys sp., Cu: Cupressus sempervirens, Ne: Nerium
oleander, Pho: Photinia fraseri, Pi: Pittosporum tobira, Pr: Prunus laurocerasus and Py: Pyracantha coccinea)
on the co-inertia Factor 1 x Factor 2 plane
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4.
Discussion
In this work, plant flammability was assessed through the measurement of physical characteristics such as the
bulk density or the proportion of each class of particle composing the plant canopy and through the measurement
of the gross heat content of these particles as well as through the measurements of the flammability variables
(ignition frequency, time-to-ignition and flaming duration) when leaves of each species were burned.
4.1. Effects of species on the proportion of particles and on their size
A two-way ANOVA was used to assess the effects of the cube location and of the species on the proportions of
the different classes of particles. These proportions were significantly influenced by the species and, except for
the 2-6mm class, by the cube location. However, there is evidence of a significant interaction between the two
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Original publication available at http://www.springer.com
doi : 10.1007/s00267-013-0067-z1616
factors, except for the class of particles >6mm. Thus, it is worth noting that, when it was significant, the cube
location had an effect that varied between the different species.
The sorting of the cubes sampled in the three locations of the plant canopy showed that the amount of
live particles, which were mainly located at the periphery of the canopy (top and centre cubes), was much greater
than that of dead particles, which were located more in depth in the canopy (base cube). Live fuel was mainly
composed of leaves and particles < 2 mm in diameter contrary to dead fuel which was mainly composed of
particles > 6 mm in diameter except for Cupressus sempervirens samples in which dead leaves dominated.
Retention of fine dead fuel within the plant structure has been observed in several Australian fire-prone species
(Specht et al. 1958, Moore and Keratis 1971, Dickinson and Kirkpatrick 1985), in the european gorse, Ulex
europeus (Baeza et al. 2006) or in the American chamise Adenostoma fasciculatum (Schwilk 2003) and fire
performance is affected by the fraction of dead fuel because of its low water content (Elvira and Hernando 1989;
Babrauskas 2003), which in ornamental vegetation is related to the age, health and maintenance of the plants.
Thus, the great amount of dead leaves within the canopy of Cupressus sempervirens would confer high
flammability to this species. Moreover, several studies have shown that fuel size affects the potential amount of
fuel that would combust in a wildfire, with fine fuel being more flammable than coarse fuel (e.g. Bond and Van
Wilgen 1996; Baeza et al. 2002) because fine particles ignite more readily and release their heat more quickly
than do thicker particles of an equivalent total weight (Wilson 1992). As the bulk density of branches and leaves,
determines the rate of oxygen flow through the fuels as well as the heat transfer between fuel elements (Scarff
and Westoby 2006), flammability mainly depends on the physical arrangement of the plant biomass (Doran et al.
2004). Thus, flammability would be high for hedges composed of species such as Pyracantha coccinea or
Phyllostachys sp which are the species with the lowest bulk densities of particles. It is worth noting that, except
for C. sempervirens and P. coccinea, the species with high proportion of a given class of particle were not the
species with high bulk density of this class.
4.2. Effect of species on flammability of leaves
Flammability of live leaves (and of dead leaves of Cupressus sempervirens) was assessed by epiradiator tests.
Live leaves of species presenting the higher moisture content had longer time-to-ignition (Nerium oleander,
Pittosporum tobira and Prunus laurocerasus) and those presenting the lowest moisture content were the most
flammable (higher ignitability and sustainability), especially live leaves of Photinia fraseri and Phyllostachys sp.
and dead leaves of Cupressus sempervirens. Currently, there is no work on the assessment of the flammability of
ornamental species using such a burning device. Data analyses showed that the species significantly affected the
different flammability variables, partly through their leaf moisture content. In addition to the burning
experiments, it would have been interesting to perform analyses of the volatile organic compounds of the leaves.
Indeed, Brown et al. (1982) showed that the flammability of fine fuel from different species may differ because
of differences in physical and chemical attributes. Contrary to Alessio et al. (2008), Ormeño et al. (2009), who
used the same burning protocol as ours, showed that the species significantly affected flaming duration but not
time-to-ignition and that ignitability was favoured by high terpene content in contrast to flaming duration.
Theses authors worked on litter beds which had lower FMC than live fuels entailing shorter time-to-ignition
values (between 2 and 4s) which were in the same range as those of dead leaves of Cupressus sempervirens
recorded in the present work. This was also the case with the work of Pellizzaro et al. (2007) who compared the
flammability of twigs of different species whose moisture contents were lower than that of leaves. Nevertheless,
regarding time-to-ignition of live leaves, our results were in the same range as those of Valette (1990) who also
worked on live leaves using the same burning protocol.
According to Valette’s classification, which takes into account ignition frequency and time-to-ignition,
the different species were ranked from slightly flammable (Pittosporum tobira) to extremely flammable
(Phyllostachys sp.); Nerium oleander and, surprisingly, live leaves of Cupressus sempervirens were not ranked
as very flammable (Table 4), even if they are known to contain great amount of volatile oils. These results
agreed with those of Etlinger and Beall (2004) who also found that Nerium oleander was not a very flammable
species, and it could therefore be recommended for planting in WUIs. In the present study, Cupressus
sempervirens displayed the longest time-to-ignition (among the species tested), contrary to the results reported
by Liodakis et al. (2002), who found that this latter species was one of the most flammable (of the species tested)
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according to its short ignition delay. This different result could be explained, as the ignition device used in these
authors’ work differed from that used in the present study. Our results also showed that flaming duration was
shortest for Prunus laurocerasus leaves and highest for Pittosporum tobira leaves what was difficult to explain
through the leaf moisture content as both species presented high foliar moisture content. In this case, analyses of
the foliar chemical content could have given more information for explaining this latter result as, according to
Behm et al. (2004), chemical components may play a role in the fire sustainability of a plant.
4.3. Effect of species on gross heat of combustion of very fine particles
Data analysis showed that the species had a significant effect on GHC of particles < 2 mm in diameter, but not
on the GHC of leaves. According to the classification proposed by Elvira and Hernando (1989), the GHC values
obtained for leaves of the species studied were generally “intermediate” (18810 - 20900 kJ kg-1), whereas the
GHC values of the particles < 2 mm in diameter were either “low” (16720 -18809 kJ kg-1) (Prunus
laurocesarus, Nerium oleander and Pittosporum tobira) or “intermediate” (Cupressus sempervirens, Pyracantha
coccinea, Photinia fraseri and Phyllostachys sp.). For each species, except for Phyllostachys sp., the GHC was
higher for leaves than for particles < 2 mm in diameter. A similar trend was observed by Dimitrakopoulos (2001)
and Nuñez-Regueira et al. (1996) in Mediterranean species and it was attributed by to the fact that leaves
constituted sites for accumulation of essential oils in plants. However, the gross heat of combustion of the
ornamental species tested in our work was generally lower than that of common Mediterranean forest fuels
(Madrigal et al. 2011). The heat of combustion of leaves of Cupressus sempervirens (19548 kJ kg-1) given by
Liodakis et al. (2002), who worked on this species as a wildland species, was lower than that of other
Mediterranean species but of the same order as those obtained in the present study (19108-19820 kJ kg-1).
4.4. Classification of species
Co-inertia analysis revealed that species with thin leaves were quick to ignite (Pyracantha coccinea,
Phyllostachys sp.) and species with high leaf GHC burned the longest (Pittosporum tobira, Nerium oleander).
This result is consistent with the findings of Behm et al. (2004), who reported species as highly consumable
when they have high energy content. Usually, flammability characteristics are mainly affected either physical
structure and physiological or cellular elements. From a physical perspective, foliage characteristics, especially
the foliage mass, foliar moisture content and surface area-to-volume ratio of fuel particles are often considered
significant factors in flammability (Fernandes and Rego 1998; Etlinger and Beall 2004). However, because we
used a radiant disk (epiradiator) as burning device, foliar mass and volume should not be considered key
variables for fire performance contrary to leaf surface area and thickness. Hierarchical classification revealed
that Pittosporum tobira was the least flammable species according to the flammability variables, (ignition
frequency, time-to-ignition and flaming duration). Dimitrakopoulos and Papaiannou (2001) ranked Nerium
oleander as a low flammability species according to its time-to-ignition. In our experiment, this species also
presented a long time-to-ignition but it was ranked as moderately flammable in the hierarchical analysis which
combined its long time-to-ignition, intermediate ignition frequency and long flaming duration.
In their fuel classification, Dimitrakopoulos and Papaiannou (2001) highlighted four clusters of species
according to their ignitability as a function of their foliage adaptations to prevent water loss. Contrary to our
results, these authors ranked Cupressus sempervirens in the cluster of flammable species because of its short
time-to-ignition due to high foliar surface area-to-volume ratio which facilitates heat absorption. Moreover, for
these authors, high flammability was linked to richness in flammable volatile essential oils, what was not the
case in the present work as none of the highly flammable species (Photinia fraseri or Phyllostachys sp.) are
known to be aromatic plant species. According to the classification system described by Valette (1990), which
takes into account only ignition frequency and time-to-ignition of leaves, and according to the results of the
hierarchical classification, live leaves of Pittosporum tobira and Cupressus sempervirens were the least
flammable (respectively rated 0 and 1, Tab. 4 and Fig. 7), particularly because of their longer time-to-ignition.
However, dead C. sempervirens was the most flammable, regarding both classifications. Thus, according to this
last result, flammability within a species can strongly vary according to the type of fuel tested (dead or live).
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This is confirmed by the work of Ganteaume et al. (2012) who assessed the flammability of the litters (dead
surface fuels mainly composed of leaves) of the same ornamental species. These authors ranked litters of
Cupressus sempervirens as moderately flammable (probably because of their high compaction), of Prunus
laurocerasus as very flammable and of Phyllostachys sp as not very flammable whereas they were ranked
respectively as not very flammable, moderately flammable and very flammable in the current work.
5.
Conclusion
In the present work, the flammability of seven ornamental species was assessed by their description at particle
level and by the measurements of flammability variables during burning experiments on leaves. Gross heat of
combustion and foliar physical characteristics helped to explain the flammability and these species were ranked
in five clusters according to the ignitability and sustainability of their leaves. Pittosporum tobira, ranked as
poorly flammable species, can be considered a fire-wise species while Phyllostachys sp. and Photinia fraseri
ranked as highly flammable, should not be planted in WUI. Regarding the flammability of its live leaves,
Cupressus sempervirens was not very flammable, however, because this species had the greatest amount of dead
material which was ranked extremely flammable, this ornamental species that should be avoided in WUI,
especially close to the houses. Further experiments, especially burning experiments of whole plants, are needed
to confirm the present results and to make them more widely applicable. Moreover, protocols for determining
flammability must be standardized, and a common classification must be established for the test results as stated
by several works (Weise et al. 2005; White and Zipperer 2010).
Although laboratory experiments help improve our knowledge of the effects of live and dead fuel
properties (species, moisture content and other physical and chemical characteristics) on flammability, the results
obtained cannot be used directly to describe or predict the flammability of fuels under natural and real
conditions; they represent basic information that is useful for assessing the fire risk of Mediterranean ornamental
vegetation planted as hedges for instance. Moreover, White and Zipperer (2010) showed that flammability
characteristics for a particular species were influenced not only by the species itself but also by their
maintenance (watering, pruning, trimming, removing dead biomass, etc.) what could reduce the potential fire
hazard. Indeed, routine irrigation, allowing the increase in FMC, has been shown to reduce plant flammability in
wildland and WUIs (Narog et al. 1991; Doran et al. 2004), and pruning of dead wood decreases fire temperature
and heat release (Schwilk 2003), indicating that plant flammability can be manipulated by horticultural practices.
Acknowledgments
The authors wish to thank the technical staff at Irstea (Roland Estève, Aminata N’Diaye, Fabien Guerra, JeanMichel Lopez, Marie Cabaret-Lampin, Florent Dalverny and Alice Lebeaux) and at INIA (Carmen Díez
Galilea).
This study was funded by the French Ministry of Ecology (Service des Risques Naturels et
Hydroliques) and the Direction Générale de la Prévention des Risques (DGPR).
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doi : 10.1007/s00267-013-0067-z1919
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Table 1 Bulk density (kg m-3) of the very fine particles (leaves and particles < 2 mm) in each cube location in the seven species studied (mean (standard deviation))
LIVE
DEAD
Base
Centre
Top
Base
Centre
Top
Cupressus
0.44 (0.44)
3.72 (2.64)
3.71 (3.34)
10.36 (11.25)
0.70 (0.62)
0.053 (0.033)
Prunus
0.30 (0.36)
3.72 (1.26)
2.28 (2.42)
0.025
0.050
-
Nerium
0.80 (0.22)
0.82 (0.78)
2.51 (2.50)
-
-
-
Pittosporum
0.14 (0.11)
1.06 (0.69)
2.28 (2.42)
0.11 (0.11)
0.025
-
Pyracantha
0.18 (0.13)
0.19 (0.077)
0.59 (0.26)
0.012
0.037 (0.018)
-
Photinia
0.38 (0.54)
0.91 (0.93)
0.98 (0.62)
-
-
-
Phyllostachys
0.12 (0.098)
0.29 (0.11)
0.52 (0.24)
-
-
-
Table 2 Results of the statistical analysis performed on the proportions of each class of particle compared across species and cube location (For the two-way ANOVAs, the
<2mm, 2-6mm and >6mm classes were transformed (log(x+1) for <2mm and >6mm classes and squareroot (x) for 2-6mm class, italic=not significant)
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Leaves
<2mm
2-6mm
>6mm
Live particles
Species
F=28.08, p<0.0001
F=35.04, p<0.0001
F=6.58, p<0.0001
F=5.85, p=0.0002
Two-way ANOVA
Cube
F=401.82, p<0.0001
F=42.41, p<0.0001
F=3.19, p=0.0512
F=52.61, p<0.0001
Interaction
F=7.48, p<0.0001
F=4.26, p=0.0002
F=3.38, p=0.0017
F=1.29, p=0.26
Dead particles
Species
KW=46.29, p<0.0001
KW=25.02, p=0.0003
KW=20.89, p=0.0019
KW=15.66, p=0.015
One-way ANOVA
Cube
KW=1.46, p=0.48
KW=3.95, p=0.14
KW=8.54, p=0.014
KW=4.19, p=0.123
23
Author-produced version of the article published in Environmental Management, 2013, vol. 52, pp 467-480
Original publication available at http://www.springer.com
doi : 10.1007/s00267-013-0067-z2424
Table 3 Leaf traits, flammability variables of live leaves (and dead leaves of Cupressus sempervirens 1) and rating according to the classification of Valette (1990) for the
species studied (Rating: 0 = slightly flammable; 1 = weakly flammable; 2 = moderately flammable; 3 = flammable; 4 = highly flammable; 5 = extremely flammable, FMC:
fuel moisture content, N1: initial number of trials; N2: number of trials with ignition)
FMC (%)
Mass (g)
Contact surface
Thickness
Volume
Surface/Volume
Ignition
Time-toFlaming
Species
area (cm2)
(cm)
(cm3)
(cm-1)
N1
frequency (%)
N2
ignition (s)
duration (s) Rating
Cupressus sempervirens
150
0.42 (0.08)
6.9 (1.10)
0.093 (0.10)
0.51 (0.12)
13.82 (1.23)
50
94 (24)
47
35.55 (6.61)
6.51 (2.72)
1
C. sempervirens (dead )
6
0.23 (0.04)
6.0 (1.10)
0.075 (0.003)
0.35 (0.06)
16.94 (0.61)
50
100
50
2.58 (0.93)
15.52 (4.02)
5
Nerium oleander
213
0.96 (0.19)
23.4 (2.5)
0.044 (0.006)
1.03 (0.22)
23.36 (3.61)
50
86 (35)
42
23.67 (6.81)
8.14 (4.20)
2
Photinia fraseri
92
0.76 (0.20)
25.3 (2.4)
0.038 (0.002)
0.95 (0.13)
26.69 (1.71)
50
100
50
14.96 (2.09)
8.26 (3.13)
4
Phyllostachys sp.
117
0.094 (0.02)
10.6 (1.8)
0.027 (0.031)
0.28 (0.31)
37.89 (1.06)
50
96 (20)
48
10.71 (4.15)
8.75 (3.18)
5
Pittosporum tobira
178
0.43 (0.05)
13.2 (1.6)
0.032 (0.002)
0.43 (0.05)
31.09 (2.10)
50
46 (50)
23
29.56 (11.73)
8.96 (6.80)
0
Prunus laurocerasus
178
1.68 (0.27)
46.6 (7.5)
0.042 (0.003)
1.98 (0.41)
23.86 (1.73)
50
98 (14)
49
17.43 (4.27)
4.92 (2.04)
4
Pyracantha coccinea
108
0.069 (0.02)
4.3 (0.5)
0.016 (0.004)
0.07 (0.02)
64.81 (11.82)
50
88 (33)
44
15.86 (5.70)
7.11 (4.99)
3
Results of statistical
KW=91.84,
KW=203.26,
KW=46.35,
tests
p<0.0001
p<0.0001
p<0.0001
1
Not taken into account in the statistical tests
24
Author-produced version of the article published in Environmental Management, 2013, vol. 52, pp 467-480
Original publication available at http://www.springer.com
doi : 10.1007/s00267-013-0067-z2525
Table 4 Gross heat of combustion (GHC, in kJkg-1) of very fine fuels in the seven species studied (Lv: leaves,
NA: no data, italic: not significant)
Species
GHC_Lv_live
GHC_2 mm_live
GHC_Lv_dead
GHC_2 mm_dead
Cupressus sempervirens
19819
19108
19265
19223
Nerium oleander
20067
18172
NA
NA
Photinia fraseri
20031
18859
NA
NA
Phyllostachys sp.
18704
19676
NA
NA
Pittosporum tobira
19929
18134
NA
NA
Prunus laurocerasus
18797
18084
NA
NA
Pyracantha coccinea
19358
18905
NA
NA
KW=12.57, p=0.0503
KW = 13.93, p = 0.0303
Results of statistical tests
25